calvinj,
I've owned dual subs several times and never thought much of it. At one point I had dual JL Audio F110s with the JL Audio CR-1 external crossover and preferred a single JL Audio F112 v2 without the CR-1. I added a second REL last night. Let me just say, it isn't the bass, but adding a second REL just opened everything up a tad more. My wife was listening to her favorite song and said for the first time she could hear fingers slide down the guitar. She went on to say it's easier to hear the music. So, if you get results like I have, as a music lover you owe it to yourself to get a second 212se.